With improved treatment and understanding of cystic fibrosis (CF), the average life expectancy of patients with this disease has increased significantly over the past two decades. Whereas life expectancy hovered around 195,6 years in the 1960's, today an individual with CF can expect to live closer to an age of 30 or even 40 years (clinical observation). However, with this increased longevity, additional metabolic abnormalities manifest requiring further medical intervention. One of these abnormalities, glucose intolerance, has been noted frequently in the past five years and some authors estimate it affects 47-75% of patients with cystic fibrosis, of which 12% convert to overt diabetes mellitus (DM). However, in assessing the glucose tolerance of 21 consecutive adult CF patients without previous manifestations of impaired glucose tolerance (IGT), we have noted an 85.7% prevalence of IGT. Recent research suggests that CF patients with glycemic derangements manifest symptoms of peripheral insulin resistance in addition to delayed or abnormal pancreatic insulin response. The mechanisms behind these metabolic derangements remain incompletely understood as there seem to be a multitude of diabetogenic processes simultaneously operating in these patients. Due to hallmark symptoms of malabsorption and under nourishment, these patients have been recommended to follow a high calorie diet, often as high as 3000-4000 Kcal per day. In an attempt to promote adequate caloric intake and yet avoid steatorrhea and other uncomfortable symptoms of fat intolerance, CF patients often consume diets high in carbohydrate. It is not unusual for these individuals to intake 60% of their calories as carbohydrates, with a high amount of these in the form of simple sugars. Traditional dietary instruction has tended to support these intake patterns. Unfortunately, this classic diet may serve to exacerbate existing underlying metabolic abnormalities.